Organisms from bacteria to humans possess mechanisms to cope with stresses that induce protein misfolding. This evolutionarily conserved Heat Shock Response allows cells to sense proteotoxic stress and quickly respond by transcriptionally activating the expression of protein folding and quality control genes. In mammals, these highly regulated events are coordinated by Heat Shock Transcription Factor 1 (HSF1), which promotes expression of stress-responsive chaperones to reduce protein misfolding and cell stress. While HSF1 has been studied for over 25 years, little is known about how cells sense cytoplasmic protein misfolding and activate HSF1. As part of the HSF1 activation mechanism, HSF1 interconverts from an inactive monomer to a DNA-binding active homo-trimer. This oligomerization event is regulated by intramolecular interactions within HSF1, post-translational modifications, and protein-protein interactions. Heat shock protein 70 (Hsp70), Hsp40, and Hsp90 have long been described to maintain HSF1 as a monomer under unstressed conditions but there is little evidence for direct interactions with HSF1. More recently, the essential eukaryotic chaperonin, TRiC, was demonstrated to negatively regulate HSF1 through a direct interaction, suggesting TRiC may directly transmit protein misfolding signals to HSF1. To understand how TRiC regulates HSF1, I outline two specific aims. In specific aim 1, I will determine the contribution of the TRiC-HSF1 interaction to the overall regulation and function of HSF1. First, I will characterize the protein interaction interface with crosslinking mass spectrometry and electron microscopy. Next, I will construct HSF1 mutants that cannot interact with, and be repressed by TRiC in vivo and evaluate how HSF1 activity changes in the absence of TRiC repression. Experiments proposed in specific aim 2 will provide mechanistic insight into how the cytosolic protein misfolding signal is sensed by TRiC and transmitted to HSF1 to activate target gene expression. I will take advantage of a small molecule activator of HSF1 that binds TRiC, HSF1A, which disrupts the TRiC-HSF1 interaction. This aim will evaluate HSF1A-mediated changes of TRiC structure and function in vitro and in vivo using substrate refolding assays and electron microscopy. Accomplishing these aims will expand our knowledge of the TRiC-HSF1 interaction and elucidate how proteotoxic stress is communicated to a central stress-protective transcription factor. As HSF1 regulation is important in human diseases ranging from neurodegeneration to cancer, a more complete understanding of this pathway could offer new avenues of treatment and therapeutic intervention.